Controlled-source electromagnetic (“CSEM”) surveys are an important geophysical tool for evaluating the presence of hydrocarbon-bearing strata within the earth. CSEM surveys typically record the electromagnetic signal induced in the earth by a source (transmitter) and measured at one or more receivers. The behavior of this signal as a function of transmitter location, frequency, and separation (offset) between transmitter and receiver can be diagnostic of rock properties associated with the presence or absence of hydrocarbons. Specifically, CSEM measurements are used to determine the spatially varying resistivity of the subsurface. A resistivity anomaly may indicate the presence of hydrocarbons in the layer exhibiting the anomaly.
In the marine environment, CSEM data are typically acquired by towing an electric dipole transmitter antenna among a number of receivers positioned on the seafloor. The receivers have multiple sensors designed to record different vector components of the electric and/or magnetic fields. Alternative configurations include stationary transmitters on the seafloor or in the water column as well as magnetic transmitter antennae. The transmitting and receiving systems typically operate independently (without any connection), so that receiver data must be synchronized with shipboard measurements of transmitter position by comparing clock times on the receivers to time from a shipboard or GPS (Global Positioning System) standard.
CSEM data are typically interpreted in the temporal frequency domain, each signal representing the response of the earth to electromagnetic energy at that temporal frequency. In raw data, the strength of each frequency component varies depending on how much energy the transmitter broadcasts and on the receiver sensitivity at that frequency. These effects are typically removed from the data prior to interpretation.
Both the phase and amplitude must be accurately determined in order to distinguish signal characteristics associated with hydrocarbons from the much larger portion of the signal that is associated to other geologic features of the subsurface.
In many examples of CSEM hardware, data cannot be effectively recorded at the nearest offsets because the dynamic range of the receiver's digitizers is too small to accurately represent the signal. This region is sometimes known as the “saturation zone” and typically encompasses source-receiver offsets of less than 500 meters.
While the phase of CSEM data can provide valuable constraints on the present or absence of hydrocarbons, in practice, phase can be difficult to measure accurately:                transmitter and receiver signals are recorded separately using different time bases (clocks) that must themselves be synchronized against a common GPS time base;        the transmitter current waveform must be accurately measured and reported from several hundreds or thousands of meters below the surface;        the responses of the receiver amplifiers must by accurately known at the frequencies where data are measured;        the receiver antennae (particularly the magnetic antennae) generally have a frequency-dependent response. Moreover, the response of the combined antennae-amplifier circuit can differ from the combined responses of the components;        small changes in the earth's resistivity close to the receiver may alter the electric and magnetic field values; and,        the chemical interaction of the transmitting antenna electrodes with the conductive and corroding seawater is not completely understood.        
Similar difficulties arise on land, although it is easier to connect both source and receivers to a common time reference. On land, phase errors occur due to localized earth inhomogeneities and, also, the problems relating to the antenna electrodes remain. The invention as disclosed herein may thus be applied to both onshore and offshore applications.
To date, phase errors have been reduced by employing high-precision and temperature-compensated clocks in the seafloor receivers. Direct measurement of the drift (time error) of these clocks relative to a time reference (such as GPS) at the start and end of the survey allows the user to stretch or compress measured data to an estimate of the reference time (S. C. Constable et al, Marine magnetotellurics for petroleum exploration Part 1: A sea-floor equipment system, Geophysics 63, 816-825 (1998)).
Also, independent receivers have been mounted to the transmitter to monitor the transmitter current that is actually injected into the water (L. M. MacGregor et al, The RAMESSES experiment—III. Controlled—source electromagnetic sounding of the Reykjanes Ridge at 57° 45′ N, Geophys. J. Int. 135, 773-789 (1998)). As before, the receiver data are corrected for the measured transmitter behavior.
Finally, laboratory measurements of the response of the receiver's amplifier-antenna system have been used to compensate field CSEM data (S. Ellingsrud, et al, Remote sensing of hydrocarbon layers by seabed logging (SBL): Results from a cruise offshore Angola, The Leading Edge 21, 972-982 (2002)).
The above methods still fail to adequately remove residual phase errors caused by clock drift, transmitter variations, and receiver calibrations. Another disadvantage associated with the methods known to date is that the combination of hardware and software needed to monitor the injected transmitter current is both costly and subject to breakdown as it must make real-time measurements while being dragged through the deep ocean.
The present invention aims to obviate or at least mitigate the above described problems and/or to provide advantages generally.